An actuator for positioning a focusing lens is installed in digital still cameras, digital video cameras, or electronic devices (e.g., cellular phones) having an image acquisition functional unit. The actuator may be a stepping motor type actuator, a piezoelectric type actuator, or a voice coil motor (VCM) type actuator.
In the VCM type actuator, a driving force may be generated in a linear direction that corresponds to a direction of a current that flows through a coil. There have been known a spring return type and a bi-directional driving type as a VCM driving type.
In a VCM having a spring return mechanism, a driving force in a first direction is generated when a driving current is supplied to the coil and a driving force in a second direction that is the opposite direction of the first direction is generated using a force of a spring installed at a mover. That is to say, both the electrical driving force and the mechanical driving force are employed. For example, when driving the VCM having the spring return mechanism, the driving current may be supplied only in a single direction of the coil so as to simplify a driving circuit.
Meanwhile, a VCM of the bi-directional driving type employs a driving circuit capable of sourcing or sinking a driving current from each ends of the VCM, such as an H-bridge circuit. In the bi-directional driving type, it is possible to switch the direction of the coil current, and thus driving forces in the positive direction and the negative direction are obtained.
FIG. 1 is a circuit diagram of a driving circuit 2r of a comparative technology that the inventor of the present disclosure has considered. The driving circuit 2r includes a logic unit 10, a D/A converter 12, and a current driver 14. The logic unit 10 receives input control data S1, which indicates a target location of a VCM 4, in a serial format from a microcomputer (not shown). The logic unit 10 generates a digital command value S2, which corresponds to the input control data S1, and outputs it to the D/A converter 12. The D/A converter 12 converts the digital command value S2 into an analog control signal S3. The current driver 14 generates a driving current IDRV according to the analog control signal S3.
Conventionally, the resolution of the D/A converter 12 is approximately 10 bits, 1024 gradations. For such D/A converter, the lower M bits are configured as an R2R type and the upper (10−M) bits are configured as a segment type, thereby securing a required precision for the resolution while suppressing an increase of a circuit area.
Recently, the resolution required for the control of the VCM has been increasing. However, if the resolution of the D/A converter is increased to e.g. 11 bits, 12 bits and more, it leads to a problem that the circuit area of the D/A converter exponentially increases.
The present disclosure has been made in order to solve such a problem. One exemplary object of an embodiment of the present disclosure is to provide a driving circuit capable of controlling a driving current with high precision, while suppressing the increase of the circuit area.